The invention relates to a process for producing neutral to alkali color developer pigments for use in carbon-free copying paper.
Carbon-free self-copying paper or reaction transfer paper has been known since the early 1950s. They are currently used in large amounts in sets of banking and shipping forms, delivery tickets, bills and so forth. Usually they consist of two or more sheets of paper on top of one another, the upper one having a color donor layer on the back (CB=coated back), the bottom one having a color acceptor layer (CF=coated front) on the front. The main component of the color donor layer includes thick-walled microcapsules of gelatin, polyurethane, melamine-formaldehyde and similar substances which contain solutions of dyes in the so-called leuco form. These only slightly colored dye precursors, predominantly from the class of di- or triphenyl methanes, thiazines, spiropyranes or fluoranes act as electron donors (Lewis bases) and can be converted into the dye form with electron acceptors (Lewis acids) in a chemical reaction. These Lewis acids are located in the color acceptor layer in the form of acid phenolic resins, zinc salicylates or acid-activated clay minerals, for example, acid-activated smectic layer silicates. If the walls of the pertinent microcapsules are destroyed by the pressure of the writing instrument when writing on self-copying paper, the contents of the capsules, the dye solution, are released and developed on the Lewis acid acceptor layer, forming a copy.
Especially suitable color developer pigments can also be produced by acid activation of clay minerals, such as attapulgites or preferably smectic phyllosilicates such a bentonites.
Preferably calcium bentonite is used, therefore a phyllosilicate with negative layer charges which are balanced by calcium ions on intermediate layer locations. In the activation process which takes place by boiling with mineral acids, usually hydrochloric acid or sulfuric acid, the bentonite is chemically changed: first, the interlamellarly bound Ca.sup.2+ is replaced in an ion exchange step by 2 H.sup.+. Second, the layer lamella is attacked from the edges and the central octahedrally coordinated Al.sup.3+, Fe.sup.2+, Fe.sup.3+ and/or Mg.sup.2+ ions, depending on the activation conditions, are more or less dissolved and washed out. As the reaction continues these polyvalent cations are partially bound again on or between the negatively charged layers. The product can thus be described as a H.sup.+ /Al.sup.3+ /Fe.sup.3+ /Mg.sup.2+ /Ca.sup.2+ bentonite with voluminous amorphous silicic acid bound to the edges. It is characterized by very high specific surfaces of roughly 300 m.sup.2 /g (measured according to the BET method), high adsorption capacity and pore volume and by the presence of many acid centers (Bronstedt and Lewis acids) on which development of the dyes proceeds catalytically.
The high-surface, acid-activated bentonite with layer structure (x-ray diffraction spectrum) which is essentially still intact, has as a phyllosilicate a pronounced lamellar structure with very high shape factor of &gt;30:1. In aqueous slurries and coating compositions compared to "normal" coating pigments such as calcium carbonate or kaolin, this causes unfavorable flow behavior, characterized by high structural viscosity and thixotropy at average solid contents of roughly 40%. For these reasons it is absolutely essential to disperse, treat, and coat acid-activated bentonites at pH of roughly 7-10 since at that range, the viscosity of these pigments is minimum. The pH is adjusted as usual by adding sodium hydroxide solution. In any case, problems often arise during dispersion since a strongly acid pigment must be distributed in an alkaline-medium. pH shocks occur which are expressed in formation of agglomerates and in thickening.
Thus, with these active pigments even with optimum coating composition preparation, pH control and use of dispersing agents, a solid content of the coating composition of roughly 45% cannot ordinarily be exceeded. This has adverse effects on dryer capacity and the running speed of the coating machine.
A sensitization pigment for color reaction recording materials is known from DE-B 2 701 056. It can be obtained by ion exchange with lithium on a dioctahedral montmorillonite. Subsequent heat treatment up to irreversible collapse of the montmorillonite structure and grinding to a particle size of less than 15 microns. Heat treatment which is done at roughly 160.degree. to 300.degree. C. destroys the laminar structure of the montmorillonite. This is manifested in a change of the x-ray diffraction spectrum in that the planar base distance of the montmorillonite decreases from 12 to 15 .ANG. to roughly 8.9 to 10.2 .ANG..
A color developer is known from EP-B-0 044 645 for pressure-sensitive copy paper. It has been obtained from a clay mineral with a laminar structure consisting of regular silica tetrahedra by acid treatment. It has an electron diffraction pattern which can be ascribed to crystals of laminar structure of regular silica acid tetrahedra which, however, in x-ray diffraction analysis does not exhibit essentially any diffraction pattern which can be ascribed to the crystals of this laminar structure and which contains as element components not only oxygen, but also silicon and magnesium and/or aluminum. In acid treatment the SiO.sub.2 content is raised to roughly 82 to 96.5% by weight, i.e., the laminar structure of the montmorillonite is largely destroyed.
A similar color developer for pressure sensitive copy paper is described in JP-A-58-217389 (Patent Abstracts of Japan, M-286, 1984, Vol. 8, No. 69). To produce this color developer one likewise proceeds from acid-treated clay minerals such a montmorillonites, with a SiO.sub.2 content of 82 to 96.5% by weight, in the x-ray diffraction spectra of which regular silica tetrahedra can no longer be ascertained. These substances are then converted with Mg and Al compounds.
A magnesium phyllosilicate is known from U.S. Pat. No. 4,830,843 which can be used as a color developer for pressure-sensitive recording papers. This phyllosilicate is produced by conversion of an active silicic acid and an active alumino-silicic acid with magnesium oxide or hydroxide under hydrothermal conditions. If one proceeds from an active alumino-silicic acid, for example, an acid-treated montmorillonite, acid treatment is conducted so far that all the aluminum is dissolved out so that it no longer appears in the formula of the magnesium silicate obtained as an end product.
A color developer for carbon-free copying paper is known from FR-A-2 203 317 which is obtained by acid activation of clays, such as saponite or hectorite. Posttreatment with alkalis does not take place.
A color developer for pressure-sensitive copy papers is known from EP-A-0 111 564 which is obtained by acid treatment of clay minerals, the SiO.sub.2 content being raised to 82 to 96.5%. The acid-activated mineral obtained is brought into contact in an aqueous medium with a soluble magnesium and/or aluminum compound, the magnesium and/or aluminum ions being incorporated into the laminar structure with exchange of hydronium ions.
Production of montmorillonite pigments with reduced charge is known from U.S. Pat. No. 4,053,324, the interlayer cations being replaced by treatment with acid by hydronium ions. The hydronium ions are then replaced by lithium ions which penetrate through the tetrahedral layer into the octahedral layer and reduce the layer charge. Acid treatment of the montmorillonite is done under comparatively mild conditions so that no more amorphous silicic acid forms on the corners and edges of the montmorillonite crystallite.
A process for producing color developer material is known from GB-A-2 051 847, a dioctahedral montmorillonite being washed in an aqueous solution with acid, whereupon the pH of the dispersion is set to roughly 7 to 11 by adding an alkali potassium compound. The montmorillonite is washed out with acid under comparatively mild conditions so that the lattice is not yet partially dissolved.
A color developer for carbon-free copy paper is known from EP-A-0 076 342 which represents a dry physical mixture of particulate activated clay with sodium silicate and optionally one or several dispersing agents. Therefore there is no reaction product present.
An adsorptively acting silicate agent for binding alkaline earth metal ions is known from DE-C-2 727 053 which has been obtained by dry or wet mixing of a material, obtained by acid activation of a mineral from the montmorillonite-beidelite series, with alkali hydroxide, carbonate, phosphate or borate. In this case the H ions on the surface and between the layers of the acid-activated material are exchanged. The agent is used first of all as a combined adsorption and water softening agent.
A color former for pressure sensitive reproduction paper and a process for its production are known from DE-B-1 809 778, in which an acid-treated dioctahedral montmorillonite clay mineral is mixed with natural dioctahedral montmorillonite clay minerals in order to produce a certain secondary color developing capacity. To further improve the color developing action alkaline substances such as sodium silicate, sodium pyrophosphate or alkaline earth metal hydroxides, for example, hydrated lime, can be added. Such an addition takes place however only in the form of physical mixing.
A similar color developer composition for pressure-sensitive recording sheets is known from EP-B-0 144 472 which contains a color developer based on a clay mineral with a laminar structure with certain properties, a metal compound in the form of oxides and/or hydroxides of calcium, magnesium and zinc, and optionally another color developer with a SiO.sub.2 content of not more than 80%. In this case as well only physical mixing of the acid clay mineral with the calcium, magnesium and zinc compound takes place.
It was surprisingly found that the viscosity and thixotropy of acid-activated smectic phyllosilicates can now be significantly improved if the acid-activated phyllosilicate is chemically converted without intermediate drying under certain condition with the basically-reacting chemicals.